Process for production of aluminum by carbothermic production of alkaline earth metal aluminide and stripping of aluminum from alkaline earth metal aluminide with sulfurous stripping agent

ABSTRACT

An improved process is disclosed for carbothermically producing an alkaline earth metal aluminide from an aluminum-bearing material which, in one aspect comprises forming a mixture of a carbonaceous reducing agent and a slag comprising the aluminum-bearing material, the alkaline earth metal compound; forming, at a first temperature, an alloy containing impurities in the slag; removing the alloy containing the impurities from the slag; and then heating the slag to a higher temperature to form the alkaline earth metal aluminide. The alkaline earth metal compound used in the process may comprise calcium carbide. Alternatively, both the alkaline earth metal compound and the aluminum-bearing material may be obtained using a calcium aluminate slag such as a byproduct from the steel industry. The calcium aluminate slag is purified in a preliminary step to remove silicon by alloying it with iron and then removing a ferrosilicon alloy formed in this step. Byproducts formed during the reactions may be recycled back is desired. 
     Either metallic aluminum, the alkaline earth metal, or both may be recovered from the aluminide material using, respectively a halide, a sulfurous, or a nitrogen stripping agent, or by reducing both metals in an electrolytic reduction cell. The alkaline earth metal aluminide may also be used as a reducing agent to recover other metals such as magnesium oxide by reduction from their respective compounds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the production of an alkaline earth metalaluminide material by a carbothermic reaction involving a reduciblealuminum-bearing compound, a second reducible metal compound, and acarbonaceous reducing agent.

2. Description of the Related Art

Conventionally, aluminum is prepared by the electrolytic reduction ofalumina in a Hall cell. However, processes involving the directreduction of aluminum-bearing salts cr oxides at high temperatures usingcarbonaceous reducing agents to form either aluminum or an aluminumalloy have been explored because of their potential for energy and costsavings.

In Cochran U.S. Pat. No. 3,971,653, there is described and claimed acarbothermic process wherein Al₂ O₃ is reacted with carbon in a firstzone to form a first liquid comprising alumina (Al₂ O₃) and aluminumcarbide (Al₄ C₃). This liquid is then. decomposed with increasedtemperature and/or decreased pressure to a second liquid comprisingaluminum and carbon. Aluminum is then recovered from this second liquid.

Persson U.S. Pat. No. 4,385,930 discloses a method for producingaluminum which comprises providing a charge of carbon in a primary zoneof an electric arc furnace containing a mixture of aluminum and aluminumcarbide and providing a charge of alumina in a secondary zone of thefurnace. The zones are heated to react aluminum with carbon to formaluminum carbide in the mixture in the first zone. The mixture flowsinto the second zone where the aluminum carbide reacts with alumina toform additional aluminum and an aluminum oxycarbide slag. This slagflows into the first zone for reaction with the aluminum carbide toproduce aluminum which is tapped out of the second zone.

Cochran et al U.S. Pat. No. 4,053,303 describes a three stepcarbothermic reduction process to form aluminum-silicon alloys. Sourcesof alumina, silica, and carbon are reacted at a temperature of 1500° to1600° C. to form silicon carbide and carbon monoxide. This mix is thenbrought to a temperature of 1600° to 1900° C. to form aluminumoxycarbide and carbon monoxide. The mix, now containing both siliconcarbide and aluminum oxycarbide, is finally brought to a temperature inthe range of 1950° to 2200° C. to produce an aluminum-silicon alloy.

The carbothermic production of an aluminum-silicon alloy is alsodescribed in European patent application Publication No. 0-097,993. Thealuminum-silicon alloy is produced from a mixture of oxides of aluminumand silicon and oxides of alkali or alkaline earth metals by reducingthe oxides using a carbon-based reducing agent in the presence of aplasma arc burner in a shaft reactor filled with reducing material. Thereduction reaction takes place at a temperature exceeding 2000° C. withliquid products consisting of an aluminum-silicon alloy collected at thebase of the shaft and the alkaline and for alkaline earth metal oxidesseparated at the top of the reactor.

Such carbothermic processes, however, involve vaporization of some ofthe aluminum formed therein as well as back reaction problems thatdecrease the amount of yield which actually can be realized inconducting such a reduction process. To address this problem, Cochran etal U.S. Pat. No. 4,299,619 taught the production of aluminum bycarbothermic reduction in a shaft-type reactor wherein an aluminumcarbide precursor is formed by reacting alumina and carbon in an upperreaction zone of the reactor. The first liquid formed therein,comprising alumina and aluminum carbide, is transferred to a lowerreaction zone to produce aluminum. Gaseous aluminum and Al₂ O vapor,which may be formed in the lower zone, may then be reclaimed in theupper zone which is maintained at a lower temperature. Any aluminumcarbide separated from the liquid in the second zone may be returned tothe first zone. In this way, at least some of the problems with regardto vaporization and back reaction may be alleviated.

Another problem which has been encountered in the production of aluminumby direct carbothermic reduction is the impurity of the aluminum productrecovered from the process. Kibby U.S. Pat. No. 4,419,126: 4,388,107;and 4,216,010 and Moore U.S. Pat. No. 4,409,021 address the problem ofcontamination of aluminum from a carbothermic process with aluminumcarbide. The aluminum carbide-bearing aluminum is reacted with analumina slag in the absence of reactive carbon. The reaction is said toproduce aluminum and carbon monoxide or an aluminum tetraoxycarbidedepending upon the reaction temperature. The patents refer to the use ofa slag which also contains CaO to reduce the reaction temperature fromabout 2000° C. (3632° F.) down to about 1500° C. (2732° F.). Twopossible modes of reaction are described. A reduction mode is said toinvolve the reduction of alumina by aluminum carbide at 2050° C. orhigher to form molten aluminum and carbon monoxide. The other mode,termed the extraction mode, is said to involve the reaction betweenalumina and aluminum carbide to form non-metallic slag compounds, suchas aluminum tetraoxycarbide.

Fijushige et al U.S. Pat. No. 4,445,934 teaches the formation ofaluminum in a single step in a blast furnace using a charge containingan alumina-containing material and a mixture of a carbon material and afluxing agent. The fluxing agent may be CaO or CaCO₃ or a mineralcontaining calcia or magnesia.

For the direct reduction of aluminum-bearing compounds with acarbonaceous material, i.e., a carbothermic process, to be economicallyattractive, the process should have minimum vaporization and backreaction losses, i.e., high yield, while producing an aluminum-bearingmaterial from which aluminum or the other metal in the material maysubsequently be recovered at a lower temperature.

SUMMARY OF THE INVENTION

We have now discovered that an alkaline earth metal aluminide materialmay be formed by the carbothermic reduction of an aluminum-bearingcompound and an alkaline earth metal compound. We have furtherdiscovered that aluminum and the alkaline earth metal may be separatedand independently recovered from the carbothermically formed aluminideat lower temperatures which therefore minimize vaporization losses whichhave characterized prior art processes.

It is therefore an object of this invention to provide an improvedprocess for the carbothermic reduction of a reducible aluminum-bearingcompound and an alkaline earth metal compound to form an alkaline earthmetal aluminide.

It is another object of this invention to provide an improved processfor the carbothermic reduction of a reducible aluminum-bearing compoundin a reaction involving a reducible alkaline earth metal compound and acarbonaceous reducing agent to form an alkaline earth metal aluminide.

It is yet another object of this invention to provide an improvedmultiple step process for the carbothermic reduction of a reduciblealuminum-bearing compound in a reaction wherein impurities are firstreduced to form an alloy at a lower temperature and then, after removalof the so-formed impurity alloy, a reducible alkaline earth metalcompound and a carbonaceous reducing agent are reacted with thealuminum-bearing compound to form an alkaline earth metal aluminide.

It is still another object of this invention to provide an improvedprocess for the carbothermic reduction of a reducible aluminum-bearingcompound in a reaction wherein calcium carbide and a carbonaceousreducing agent are reacted with the aluminum-bearing compound to formcalcium aluminide.

It is a further object of this invention to provide an improved processfor the carbothermic reduction of a reducible aluminum-bearing compoundcomprising an impure calcium aluminate slag wherein impurities,including silica, are removed by reacting the slag at a firsttemperature with a carbonaceous reducing agent and iron or iron oxide toform a ferrosilicon alloy and then, after removal of the so-formedferrosilicon alloy and other reduced impurities, the carbonaceousreducing agent is further reacted at a higher temperature with thecalcium aluminate slag to form calcium aluminide.

It is yet a further object of this invention to provide an improvedprocess for the production and recovery of aluminum wherein an alkalineearth metal aluminide is first formed by the carbothermic reduction of areducible aluminum-bearing compound and a reducible alkaline earth metalcompound in a reaction with a carbonaceous reducing agent; and then thealkaline earth metal aluminide is contacted with a halide strippingagent to form metallic aluminum and an alkaline earth metal halide.

It is a still further object of this invention to provide an improvedprocess for the production and recovery of aluminum wherein an alkalineearth metal aluminide is first formed by the carbothermic reduction of areducible aluminum-bearing compound and a reducible alkaline earth metalcompound in a reaction with a carbonaceous reducing agent; and then thealkaline earth metal aluminide is contacted with a sulfurous strippingagent to form metallic aluminum and an alkaline earth metal sulfide.

It is another object of this invention to provide an improved processfor the production and recovery of aluminum and a alkaline earth metalwherein an alkaline earth metal aluminide is first formed by thecarbothermic reduction of a reducible aluminum-bearing compound and areducible alkaline earth metal compound in a reaction with acarbonaceous reducing agent; and the alkaline earth metal aluminide isthen placed in an electrolytic reduction cell to form both aluminum andthe alkaline earth metal in metallic form which may then be recoveredfrom the cell.

It is yet another object of this invention to provide an improvedprocess for the production and recovery of an alkaline earth metalwherein an alkaline earth metal aluminide is first formed by thecarbothermic reduction of a reducible aluminum-bearing compound and areducible alkaline earth metal compound in a reaction with acarbonaceous reducing agent; and then the alkaline earth metal aluminideis contacted with a nitrogen stripping agent to form aluminum nitrideand the alkaline earth metal.

It is a still further object of this invention to provide an improvedprocess for the production and recovery of aluminum wherein an alkalineearth metal aluminide is first formed by the carbothermic reduction of areducible aluminum-bearing compound and a reducible alkaline earth metalcompound in a reaction with a carbonaceous reducing agent; the alkalineearth metal aluminide is contacted with a stripping agent to formmetallic aluminum and an alkaline earth metal compound; and at least thealkaline earth metal values from the alkaline earth metal compound arerecycled back to the original reaction.

It is another object to provide an improved process wherein an alkalineearth metal aluminide, first formed by the carbothermic reduction of areducible aluminum-bearing compound and a reducible alkaline earth metalcompound in a reaction with a carbonaceous reducing agent, is used as areducing agent to recover one or more other metals from theircorresponding metal compounds by reaction therewith to reduce the metalin the metal compound.

These and other objects of the invention will be apparent from theaccompanying description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet illustrating the process of the invention.

FIG. 2 is a schematic, vertical cross-sectional view of a reactor whichmay be used in the practice of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The improved process of the invention comprises the production of analkaline earth metal aluminide from various aluminum and alkaline earthmetal sources by carbothermic reduction, the use of such an aluminidematerial, and the recovery of aluminum and/or the alkaline earth metalfrom this aluminide material.

a. Production of Alkaline Earth Metal Aluminide

In the practice of the invention, an aluminum-bearing material ischarged to a reactor 10, as shown in FIG. 2, together with an alkalineearth metal compound and a carbonaceous reducing agent. The ratio of thealuminum-bearing material to the alkaline metal compound should becontrolled to promote the formation of an alkaline earth metaldialuminide such as CaAl₂ in preference to tetraaluminide such as CaAl₄since, for example, CaAl₂ is thought to inhibit formation of undesirablealuminum carbides more than would CaAl₄. This may be done by providing aratio which will give a stoichiometric ratio of aluminum to alkalineearth metal just slightly in excess of that needed to form thedialuminide.

Heat can be supplied to reactor 10 using an electric arc, a plasma, orusing other conventional heat sources which may include burning some ofthe carbonaceous material as fuel in a blast furnace.

The reactants are heated to an initial temperature of from about 1000°C., preferably about 1700° C., to just below 1900° C., preferably about1850° C. At this temperature, a slag layer 30, containing oxides of thealkaline earth metal and aluminum, is formed. However, impuritiescommonly found in the reactants, such as silicon, iron, and titanium,will react with the carbonaceous reducing agent at this temperature toform a ferrosilicon-titanium alloy layer 40 which can then be removedfrom the reaction mass. This ferrosilicon-titanium alloy may alsocontain other reduced impurities such as manganese, copper, and zinc ifmetal compound or through the use of an impure form of the carbonaceousreducing agent.

The ferrosilicon-titanium alloy containing other reduced impuritieswhich is formed during this purification step is immiscible with theslag at temperatures above 1600° C. and will sink to the bottom of thereactor where it may then be removed from the reaction mass by anyacceptable means such as by draining the alloy from reactor 10 via anexit port 20 shown at the bottom of reactor 10.

The remaining reactants, which, at this point should comprise thealkaline earth metal oxide/aluminum oxide slag plus at least a portionof the initial charge of carbonaceous reducing agent with all or most ofthe impurities removed, are now further heated to a higher temperatureof from about 2000° to 2100° C., preferably about 2050° C. to reduceboth the aluminum oxide and the alkaline earth metal oxide to form thedesired alkaline earth metal aluminide.

It should be noted here that all of the carbonaceous reducing agent maybe added initially before the purification step unless the initialpresence of sufficient carbonaceous reducing agent for both stepspromotes the formation, with some impurities, of carbides which cannotbe removed with the ferrosilicon alloy at the end of the purificationstep. It is preferred to add all of the carbonaceous reducing agentprior to the initial purification step because a less pure reducingagent can then be utilized and impurities in the carbonaceous reducingagent can also be removed during the purification step.

In one embodiment, the aluminum-bearing material and the alkaline earthmetal compound may both comprise oxides of the respective metals. Theoxides may be in either a pure or impure form. Calcium oxide (lime) is aparticularly preferred reactant in view of its availability and cost.

The reaction equations, when calcium oxide is used as the alkaline earthmetal compound, may be written as:

    Al.sub.2 O.sub.3 +4C+CaO→CaAl.sub.2 +4CO

and/or

    2Al.sub.2 O.sub.3 +7C+CaO→CaAl.sub.4 +7CO

In either case, it will be seen that the only other reaction productformed in addition to the alkaline earth metal aluminide, in thisinstance calcium aluminide, is carbon monoxide gas. Thus by carrying outthe separation of impurities after the initial reaction step, a fairlypure alkaline earth metal aluminide product may be obtained. Thisproduct, as will be discussed below, may be used as a reducing agent orthe aluminide constituents may be separated to recover either or both ofthe metals as will also be described below.

In one embodiment of the invention, the alkaline earth metal compoundmay comprise calcium carbide which may be commercially purchased andtherefore be used as a purified material. If a pure calcium carbidereactant is used, a more purified form of an aluminum-bearing compound,such as alumina, may also be used as well as a purified source ofcarbon. The calcium aluminide may then be produced by reacting thematerials at a temperature of from about 1900° to 2000° C. The use ofthe terms "purified" and "pure" are intended to mean a reactant whichcontains no more than 1 wt. % impurities, other than Ca containingcompounds which are present in CaC₂.

The reaction equations, when calcium carbide is used as the alkalineearth metal compound, are:

    Al.sub.2 O.sub.3 +CaC.sub.2 +C→CaAl.sub.2 +3CO

and/or

    2Al.sub.2 O.sub.3 +CaC.sub.2 4C→CaAl.sub.4 +6CO

It will be noted that an additional source of carbonaceous reducingagent is needed despite the presence of carbon in the calcium source.The ratio of the amounts of pure calcium carbide, alumina, and carbonwhich should be used in the reaction are 10-40 mole % CaC₂, 25-40 mole %Al₂ O₃, and 30-60 mole % carbon.

The source of the aluminum-bearing material and the alkaline earth metalcompound may also be a calcium aluminate slag such as found as abyproduct of the steel-making industry. This material, however, willcontain impurities including silica and other silicon compounds. Thesilica, as well as other impurities present in the slag, may be removed,however, by first reacting the calcium aluminate slag at a temperatureof from about 1600° to 1900° C. with enough carbonaceous reducing agentand sufficient iron or iron oxide to form a ferrosilicon alloycontaining less than 20 wt. % silicon which is then removed from theremaining slag.

The amount of iron added to the slag is crucial since silicon will formsilicon carbide in the presence of carbon if the silicon content in theferrosilicon alloy exceeds at 20 wt. %. Therefore, since the amount ofsilica usually found in such slag may be as high as 42 wt. %, it isusually prudent to react the slag with carbon and iron in a ratio ofabout 51 wt. % slag to at least about 40 wt. % iron and 9 wt. % carbon,or with carbon and iron oxide, in a ratio of about 39 wt. % slag to atleast 44 wt. % Fe₂ O₃ and 17 wt. % carbon to ensure substantiallycomplete removal of the silica in the slag.

After formation of the ferrosilicon alloy, the alloy may be removed fromreactor 10 using exit port 20 at the bottom of reactor 10 as previouslydescribed with respect to the ferrosilicon-titanium alloy.

b. Stripping of Alkaline Earth Metal Aluminide to Recover One or BothMetals

The alkaline earth metal aluminide, e.g., calcium aluminide, produced inaccordance with the invention may be subsequently reacted with strippingagents to recover either or both metals.

Aluminum metal may be recovered from the alkaline earth metal aluminideby contacting the material with a halogen-containing material. Suchhalogen-containing stripping agents include F₂, Cl₂, Br₂, I₂, HF, HCl,HBr, HI, AlF₃, AlCl₃, AlBr₃, and AlI₃. Carbohalides represented by theformula RX_(n) may also be used as the stripping agent to recoveraluminum from the alkaline earth metal aluminide wherein R equals a 1-4carbon chain; X equals F, Cl, Br, or I; and n equals an integer from 1to 4 when R equals 1 carbon, and an integer from 1 to 6 when R equals 2or more carbons.

When the halide stripping agent is in the form of a gas, it may bebubbled through the heated aluminide material while maintaining thealuminide material at a temperature of from above 540° C. to 1100° C.,depending upon the composition. When the halide stripping agent is asolid or a liquid, it may be added to the aluminide material prior toheating followed by heating of the mixture to a temperature of fromabove 540° C. to 1100° C., depending upon the composition, andmaintaining the reaction mass at this temperature for at least about 30minutes. The amount of the halogen stripping agent which is mixed withthe alkaline earth metal aluminide should be in excess of the amountneeded to react with all of the alkaline earth metal present in thealuminide material to ensure production of nominally pure aluminum.

The alkaline earth metal in the aluminide material will react with thecorresponding halide to form the halide of the alkaline earth metal andmetallic aluminum. When a carbohalide is used, the carbonaceous portionof the stripping compound will be vaporized and removed throughappropriate venting means over the reaction vessel. The metallicaluminum, as it forms, will sink to the bottom of the reaction vesselwhere it may be easily removed from the alkaline earth metal halide, forexample, through exit port 20 in reactor 10 shown in FIG. 2.

The alkaline earth metal aluminide may also be reacted with a sulfurousstripping agent to recover metallic aluminum from the aluminidematerial. The sulfurous stripping agent may comprise elemental sulfur,H₂ S, A1₂ S₃, or other metal sulfides such as ferrous disulfide (FeS₂).

Sulfur was tested as a stripping agent. Calcium: aluminide (23.5 g) andsulfur (10 g) were mixed and heated to 750° C. in vacuum for two hours.X-ray diffraction patterns showed the resulting products were Al andCaS, and coalescence of the aluminum was good.

Al₂ S₃ was also tested. A mixture of calcium aluminide (131 g) and Al₂S₃ (83 g) was heated to 1125° C. for two hours. A small quantity ofaluminum was produced, but the reaction was slow, and a considerablequantity of the calcium aluminide remained unreacted.

The stripping reaction is carried out by mixing the sulfurous strippingagent with the aluminide material in a mole ratio of at least 1 moleequivalent of stripping agent per mole of alkaline earth metal aluminideto provide sufficient sulfur to react with all of the alkaline earthmetal present in the aluminide material to form metallic aluminum and analkaline earth metal sulfide. The mixture is heated to of above 540° C.up to 1100° C. and maintained at this temperature for a period of atleast about 30 minutes to provide for complete stripping of the alkalineearth metal from the aluminide material and reaction to form thesulfide. Metallic aluminum, which will sink to the bottom of thereactor, may then be drained through an appropriate port at the bottomof the reactor as previously described.

The aluminum and alkaline earth metal values may both be recovered fromthe alkaline earth metal aluminide using an electrolytic process. Inthis embodiment of the invention, the alkaline earth metal aluminide isplaced in an electrolytic cell where it functions as the anode andalkaline earth metal, separated from the aluminide material, functionsas the cathode. Advantageously, the molten aluminide material is placedin a fused bath cell as the bottom layer beneath the fused bath layerwherein the alkaline earth metal is electrolytically transported throughthe middle fused bath layer to form an upper layer of molten alkalineearth metal which functions as the cathode of the cell. When all of thealkaline earth metal in the aluminide material is electrolyticallytransported to the cathode layer, aluminum may be recovered from theanode layer and the alkaline earth metal may be recovered from thecathode layer.

A typical fused salt bath which can be used for this type of reactioncomprises 0-100 wt. % calcium chloride and 100-0 wt. % potassiumchloride. Typically the reaction is carried out at a temperature of atleast 1080° C., which can be dropped to about 800° C. as the amount ofcalcium in the bath is lowered, using a current of about 1 to 20amps/cm² and a voltage of about 1 to 10 volts to transport substantiallyall of the alkaline earth metal to the cathode layer.

To initiate the cell reaction, a layer of alkaline earth metal may beplaced in the bath to function as the cathode during initiation of theelectrolytic cell reduction. Aluminum may also be added to the cell uponstart-up if found necessary.

If it is desired principally to recover only alkaline earth metal fromthe alkaline earth metal aluminide rather than aluminum or both alkalineearth metal and aluminum, or if the production and recovery of aluminumnitride is desired, the aluminide material may be reacted with anitrogen gas whereby the aluminum is stripped from the aluminidematerial to form AlN leaving behind the reduced alkaline earth metal.This may be accomplished by heating the aluminide material to atemperature of at least 1100° C. and bubbling sufficient dry N_(`)through the heated aluminide material to strip all of the aluminum fromthe aluminide material leaving behind a layer of molten alkaline earthmetal floating over the solid aluminum nitride. The temperature can bedropped to about 900° C. as most of the aluminum is removed. Calcium,barium, magnesium, and strontium alkaline earth metals (and marginallyberyllium) may be recovered in this manner.

The following equations illustrate an example of a particular process inaccordance with the invention wherein metal sulfides, particularlyferrous disulfide, may be used as stripping agents to remove and recovermetallic aluminum from calcium aluminide formed in accordance with theinvention wherein the byproducts, particularly byproducts containingaluminum and calcium values, can be recycled back to the originalreduction products.

    (6CaO+6Al.sub.2 O.sub.3)+24C→6CaAl.sub.2 24CO       (1)

    8Al.sub.2 O.sub.3 6C+FeS.sub.2 →(6Al.sub.2 O.sub.3 +2Al.sub.2 S.sub.3)+3Fe+6CO                                          (2)

    (6Al.sub.2 O.sub.3 +2Al.sub.2 S.sub.3)+6CaAl.sub.2 →16Al+(6Al.sub.2 O.sub.3 +6CaS)                                            (3)

    (6Al.sub.2 O.sub.3 +6CaS)+6H.sub.2 O+6CO.sub.2 →(6Al.sub.2 O.sub.3 +6CaCO.sub.3)+6H.sub.2 S                                  (4)

    (6Al.sub.2 O.sub.3 +6CaCO.sub.3)→(6CaO+6Al.sub.2 O.sub.3)+6CO.sub.2 (5)

In this set of reactions, the original formation of the calciumaluminide, which occurs at a temperature range of about 2000° C. to2100° C., is illustrated in equation (1) while the formation of aluminumsulfide and iron from alumina and ferrous disulfide, at a temperatureequivalent to at least about 1600° C. at 1 atmosphere pressure, (theHaglund Process) is shown in equation (2). The process shown in equation2 may be carried out at higher or lower temperatures by varying thepressure of the reaction. The alumina/aluminum sulfide material inparentheses on the right side of equation (2) may then be reacted inequation (3), at a temperature of about 540° to 1100° C., with thecalcium aluminide produced in equation (1) to form metallic aluminum andthe mixture of alumina and calcium sulfide shown in parentheses on theright side of equation (3). This mixture of alumina and calcium sulfidefrom equation (3) may then be reacted, in equation (4), at a temperatureof at least about 500° C., with water and carbon dioxide to formhydrogen sulfide and a mixture of alumina and calcium carbonate shown inparentheses on the right side of equation (4). This mixture of aluminaand calcium carbonate from equation (4) may then be heated and reacted,in equation (5), to over 900° C. to form carbon dioxide and a mixture ofalumina and calcium oxide, i.e., the initial reactants in equation (1)to which these reaction products may then be recycled.

In this embodiment, alumina, carbon, and ferrous disulfide are the onlyreactants which must be added to the process stream and both iron andaluminum may be recovered from the process. Ferrous selenide or ferroustelluride could be substituted for the ferrous disulfide in the aboveequations to also recover both iron and aluminum.

The following equations illustrate another example of a particularprocess, in accordance with the invention, wherein calcium aluminide,and subsequently metallic aluminum, is formed wherein the byproducts,particularly byproducts containing aluminum and calcium values, can berecycled back to the original reduction products.

    4Al.sub.2 O.sub.3 +12C+2CaX.sub.2 →2CaAl.sub.2 +4AlX+12CO (1)

    4AlX+4CO→4/3AlX.sub.3 +4/3Al.sub.2 O.sub.3 +4C      (2)

    2CaAl.sub.2 +4/3AlX.sub.3 →2CaX.sub.2 +16/3Al       (3)

In these reactions, aluminum oxide (alumina) and a calcium halide suchas calcium chloride may be reacted, as shown in equation (1), withcarbon at a temperature of at least the equivalent, at 1 atmosphere, ofabout 1975° C. to form calcium aluminide, an aluminum monohalide vaporand carbon monoxide gas in a first reaction chamber.

The aluminum monohalide and the carbon monoxide gases formed in thisreaction may then be transported to a second, cooler, reaction chamberwhere they may be further reacted at a lower temperature, of no morethan the equivalent, at 1 atmosphere, of about 1325° C. to form thealuminum trihalide, alumina, and carbon, as shown in equation (2).

The alumina and carbon from the reaction shown in equation (2) may berecycled back to the reaction of equation (1). The aluminum trihalidefrom the reaction of equation (2) and the calcium aluminide from thereaction in equation (1) may then be reacted together in the reactionshown in equation (3) to produce metallic aluminum and calcium halide.This, for example, may be accomplished by circulating the aluminumtrihalide produced in equation (2) back to the initial reaction chargerin which the calcium aluminide was formed. The calcium halide formed inthe reaction of equation (3) may be then recycled back to the reactionof equation (1).

In this set of reactions, the only feed materials may be alumina andcarbon and the only products are metallic aluminum and carbon monoxidewith the remainder of the byproducts all recycled back into the processloop.

c. Alkaline Earth Metal Aluminide as Reducing Agent

As described above, the alkaline earth metal aluminide, e.g., calciumaluminide, produced in accordance with the invention may be subsequentlyreacted with stripping agents to recover either metal orelectrolytically refined to produce both metals. Alternatively, however,the aluminide material may be used as a valuable reducing agent for therecovery of other metals from their corresponding metal compounds, e.g.,from an oxide of the metal, in accordance with the following equationusing calcium aluminide as an example of the reducing agent and R as themetal to be reduced:

    CaAl.sub.2 +4RO→4R+Al.sub.2 O.sub.3 +CaO

As an example of this type of reduction reaction, calcium aluminide maybe used as a reducing agent to recover magnesium metal from magnesiumoxide in accordance with the following equation:

    CaAl.sub.2 +4MgO→4Mg.sub.(v) +CaO..sub.Al2 O.sub.3

The reduction is carried out at a temperature range of from about 1200°C. to about 1700° C. in an oxygen-free atmosphere at a pressure of aboutone atmosphere for a time period sufficient to reduce substantially allof the metal compound.

Examples of other metal compounds which can be reduced in accordancewith the above equation include oxides of the alkali metals such aslithium oxide, sodium oxide, potassium oxide, rubidium oxide, and cesiumoxide.

Metal compounds such as those listed above may be reduced by reactionwith the alkaline earth metal aluminide at lower temperatures, i.e.,below 1200° C., to form an alloy of aluminum and the reduced metalinstead of vaporizing in accordance with the following equation, againusing calcium aluminide as an example of the alkaline earth metalaluminide reducing agent:

    CaAl.sub.2 +R'O→R'+Al+CaO

Thus the invention provides an improved process for the carbothermicreduction and recovery of aluminum and/or alkaline earth metal values inan efficient manner which conserves energy without compromising purityor yield.

Having thus described the invention, what is claimed is:
 1. An improvedcarbothermic process for producing aluminum from an alkaline earth metalaluminide carbotherically formed by reaction of an aluminum-bearingmaterial and an alkaline earth metal compound with a carbonaceousreducing agent which comprises the steps of:(a) forming a slagcomprising said alkaline earth metal compound and said aluminum-bearingmaterial; (b) reducing said slag with a reducing agent to form thecorresponding alkaline earth metal aluminide: and (c) reacting saidalkaline earth metal aluminide with a sulfurous stripping agent to formmetallic aluminum and an alkaline earth metal sulfide.
 2. A process inaccordance with claim 1 wherein said step of reacting said alkalineearth metal aluminide with a sulfurous stripping agent comprisesreacting said alkaline earth metal aluminide with a sulfurous strippingagent selected from the class consisting of H₂ S, Al₂ S₃, and FeS₂. 3.The process of claim 1 wherein said step of reacting said alkaline earthmetal aluminide with said sulfurous stripping agent comprises addingsaid stripping agent to said alkaline earth metal aluminide and thenmaintaining said mixture at a temperature of from at least 540° C. to1100° C. for a time period sufficient to substantially react all of saidalkaline earth metal aluminide with said sulfurous stripping agent toform said metallic aluminum.
 4. The process of claim 1 wherein theamount of said sulfurous stripping agent which is added to said alkalineearth metal aluminide is in excess of the amount needed to react withall of the alkaline earth metal present in said aluminide to ensurecomplete recovery of all of the aluminum values in said alkaline earthmetal aluminide.
 5. An improved process for carbothermically producingaluminum from an aluminum-bearing material comprising the steps of:(a)reacting said aluminum-bearing material with a source of carbon andcalcium to form a calcium aluminate-containing slag; (b) heating saidslag to form calcium aluminide: (c) reacting said calcium aluminide witha sulfurous material to form metallic aluminum and calcium sulfide; and(d) separating said metallic aluminum from said calcium sulfide.
 6. Theprocess in accordance with claim 5 wherein said step of reacting saidcalcium aluminide with a sulfurous r:aterial comprises reacting saidcalcium aluminide with a sulfurous material selected from the classconsisting of H₂ S, Al₂ S₃, and FeS₂.
 7. The process in accordance withclaim 6 wherein said step of reacting said calcium aluminide with asulfurous material to form metallic aluminum and calcium sulfide furthercomprises reacting said calcium aluminide with said sulfurous materialin molten salt form.
 8. The process in accordance with claim 7 whereinsaid sulfurous material is mixed with said calcium aluminide in anamount sufficient to provide at least one mole equivalent of sulfurousmaterial per mole of calcium aluminide.
 9. The process in accordancewith claim 6 wherein said step of reacting said calcium aluminide with asulfurous material to form metallic aluminum and calcium sulfide furthercomprises reacting said calcium aluminide with said sulfurous materialby bubbling said sulfurous material through said calcium aluminide. 10.The process of claim 9 wherein said sulfurous material is bubbledthrough said calcium aluminide at a temperature sufficient to permitsaid material to react with said calcium aluminide to form said metallicaluminum.
 11. The process of claim 10 wherein said step of bubbling saidsulfurous material through said calcium aluminide further comprisesmaintaining said calcium aluminide at a temperature between about 540°C. to 1100° C. while bubbling said sulfurous material therethrough. 12.An improved process for carbothermically producing aluminum from analkaline earth metal aluminide formed from reacting an aluminum-bearingmaterial and an alkaline earth metal compound with a carbonaceousreducing agent which comprises the steps of:(a) forming a slagcomprising said alkaline earth metal compound, said aluminum-bearingmaterial, and said carbonaceous reducing agent at a first temperaturesufficiently high to permit reduction of impurities in said reactants;(b) removing said reduced impurities from said slag: (c) raising saidtemperature to a second temperature sufficiently high to permit saidalkaline earth metal compound and said aluminum-bearing material toreact with said carbonaceous reducing agent to form said alkaline earthmetal aluminide; and (d) forming metallic aluminum by contacting saidalkaline earth metal aluminide with a sulfurous stripping agent selectedfrom the class consisting of H₂ S, Al₂ S₃, and FeS₂.
 13. The process inaccordance with claim 12 wherein said sulfurous stripping agent is mixedwith said calcium aluminide in an amount sufficient to provide at leastone mole equivalent of sulfurous stripping agent per mole of calciumaluminide.
 14. A process in accordance with claim 13 wherein said slagforming step is carried out at a temperature of from about 1700° to justunder 1900° C. to permit reduction of metal-bearing impurities in saidslag to form a removable metal alloy.
 15. A process in accordance withclaim 14 wherein said step of raising said temperature to a secondtemperature sufficiently high to permit reaction to form said alkalineearth metal aluminide comprises raising the temperature to from about2000° C. to about 2100° C.
 16. The process of claim 15 wherein saidalumirum-bearing material comprises an aluminum oxide.
 17. The processof claim 15 wherein said alkaline earth metal compound consistsessentially of an alkaline earth metal oxide selected from the classconsisting of barium oxide, calcium oxide, and strontium oxide.
 18. Theprocess of claim 15 wherein said carbonaceous reducing agent consistsessentially of carbon.
 19. In a process for carbothermically producingaluminum from a calcium aluminide formed by reacting a reduciblealuminum-bearing material with a source of carbon and calcium oxide, theimprovement which comprises removing reducible metal-bearing impuritiesfrom the reducible aluminum-bearing material by the steps of heatingsaid reducible aluminum-bearing material in the presence of said calciumoxide and carbon at a temperature sufficient to reduce said reduciblemetal-bearing impurities in said reducible aluminum-bearing material toone or more metal alloys without substantially reducing said reduciblealuminum-bearing material; separating said one or more reduced metalalloys from said reducible aluminum-bearing material before forming saidcalcium aluminide; and then recovering metallic aluminum from saidcalcium aluminide by reacting said calcium aluminide with a sulfurousstripping agent selected from the class consisting of H₂ S, Al₂ S₃, andFeS₂.
 20. The process in accordance with claim 19 wherein said sulfurousstripping agent is mixed with said calcium aluminide in an amountsufficient to provide at least one mole equivalent of sulfurousstripping agent per mole of calcium aluminide.